The present invention relates to a method and apparatus for brazing, and particularly the brazing of surface components onto a printed substrate such as a circuit board.
The difference between a conventional through-component and a surface component lies in the manner of securing each of these components to the printed circuit board.
The through-component has tongues or prongs which pass through the circuit board in the desired positions, and are fixed therein by brazing, such as for example, by single or double waves of a melted brazing alloy.
The surface component is secured on the surface of the circuit board by another type of brazing, this being effected by means of a brazing paste constituted by microballs of brazing alloy and secured by a flux giving a pasty texture. The brazing cream is conventionally applied on the circuit board by serigraphy, or by other known processes, thereby forming contact points with the surface components. The surface components are oppositely arranged, and the temperature of the circuit board is raised to a temperature higher than the melting temperature of the brazing alloy.
The post-brazing problems of through and surface components are different. With regard to brazing the through-components, there must be an undesirable excess amount of brazing compound or an excess amount of gaseous bubbles contained in the brazes, which can be remedied either by application of a hot jet of fluid (either gas or liquid) first to melt and then to dispel the excess brazing compound, or by melting the brazes to permit the gas bubbles to escape. With regard to brazing surface components with the aid of a brazing cream, there must be a multitude of microballs dispersed in a random fashion on the circuit board. These microballs are of different sizes and originate from the brazing alloy constituting the microballs of the brazing cream
The practical solutions for overcoming these problems of post-brazing the through-components are not effective to make the microballs disappear. That is, the application of a hot jet of fluid on the circuit board, first to melt the microballs and then to dispel them from the card, results in the simultaneous melting of the brazes and the displacement or ejection of the surface components themselves.
Remelting of these microballs without the application of mechanical force, as in the case of a jet of hot fluid, can neither eject them from the card nor integrate them to the metallic portions (lines and spaces for reception) of the circuit of the circuit board. The reason why a remelted microball in contact with a metallic surface cannot integrate itself with the metallic surface is the same reason why this same microball is not able to integrate itself to the metallic surface during brazing. That is, the metallic surface and/or the microball is contaminated. This contamination results from the fact that the surface of the microball and/or the metallic surface contain or carry a material, for example, an oxide or denatured flux, which prevents these melted microballs from spreading onto the metallic surface even though melted. Thus, the natural tendency of a melted metallic microball located on a metallic surface, whether melted or not, is to spread out on the surface to integrate therewith, thereby minimizing the exposed surface of the ensemble. The contamination of one or both surfaces prevents this reunion.
It should be noted that the conductors are often covered by a metallic alloy comparable to the brazing alloy from a standpoint of its composition and melting temperature. A metallic surface on a circuit board may be either a conductor or a reception site or any part of the component.